Diverse industrial applications are being sought by scientists and engineers for ceramic materials which exhibit superconducting properties (conduct electrically with no resistance) at relatively high superconductive transition temperatures, T.sub.c, such as above the boiling point of liquid nitrogen (77K). Ceramic materials synthesized from yttrium, barium and copper oxides in 1:2:3 molar ratio, such as YBa.sub.2 Cu.sub.3 O.sub.x (also known as Ba.sub.2 YCu.sub.3 O.sub.x) material with x being from 6 to 7, exhibit relatively high superconductive transition temperature, T.sub.c. For example, see P. M. Grant et al., "Superconductivity above 90K in the compound YBa.sub.2 Cu.sub.3 O.sub.x : Structural, transport and magnetic properties." Physical Review B, Vol 35, No. 13, May 1, 1987, pp. 7242-7244. Among these, YBa.sub.2 Cu.sub.3 O.sub.7, a perovskite superconductor which can have T.sub.c of 90K is deemed the most likely composition to achieve industrial application.
A solid state reaction process suitable for forming YBa.sub.2 Cu.sub.3 O.sub.7 containing powders on a relatively large scale is described by R. B. Poeppel et al. in "Fabrication of YBa.sub.2 Cu.sub.3 O.sub.7 Superconducting Ceramics", Chemistry of High-Temperature Superconductors, D. L. Nelson, M. S. Whittingham and T. F. George, eds., American Chemical Society, Washington, D.C., 1987, pp. 261-265. The published process comprises wet milling raw powders of BaCO.sub.3, Y.sub.2 O.sub.3 and CuO.sub.x, evaporating excess fluid after the milling step, and calcining the resultant milled raw powder. The calcining is conducted in one of two variants: (a) a long-term precalcine of raw powder at 850.degree. C. for a period of 24 h, followed by lightly grinding, quick heat-up (in about 15 min.) to 950.degree. C. and final-calcining at 950.degree. C. for about 2 hrs., or (b) heating the raw powders at 950.degree. C. for 2-6 h, cooling, regrinding and repeating this procedure for three calcinations. The calcined powder is dry pressed with the addition of suitable binders into a desired green form, e.g. disk, tape, wire, etc., which is sintered by preheating to above 850.degree. C., heating at 975.degree. C. and cooling. Unfortunately, such a process of preparing calcined powders has at least the following disadvantages:
(1) The step of evaporating excess fluid from the milled slurry may lead to segregation of component particles through disproportionate sedimentation of components with higher densities, especially if these components are present as relatively large agglomerates (relative to the size of the individual particles). This may lead to formation of several other phases in addition to Ba.sub.2 YCu.sub.3 O.sub.7 as well as formation of BaCO.sub.3 --CuO eutectic.
(2) The calcining process includes multiple calcining with intermediate granulation. Multiple calcining and intermediate grinding are time and effort consuming and often add impurities. Some of the calcining is conducted at temperatures, e.g. 950.degree. C., which are conducive to production of larger particles and melting resulting in formation of hard aggregates which may be granulated with difficulty, if at all.
Subsequent steps of remilling, recalcining, granulating and sintering often do not homogenize these powders sufficiently to result in sintered bodies of substantially homogeneous, essentially single phase material on a consistently reproducible basis.
An attempt by the present inventors to produce powders utilizing the process of R. B. Poeppel et al., but with a sequence of filtering, drying and granulating steps (hereinafter being referred to as "filter-drying") in place of the evaporating step led to powders having several other phases in addition to Ba.sub.2 YCu.sub.3 O.sub.7. Batches of mixed raw materials processed by filter-drying did not calcine into .gtoreq.95% single phase powder. It was concluded that the process including the filter-drying of wet milled slurry cannot yield powders with a sharp superconducting transition. Thus, it is still desirable to design a simple, easily controllable and efficient process for producing powders of substantially homogeneous, .gtoreq.95% single phase Ba.sub.2 YCu.sub.3 O.sub.7 which could consistently result, upon sintering, in bodies of substantially single phase material.